It works on US bands! I have one! I've been able to send SMS and make phone calls fine on Mint Mobile.

(Still haven't had time to fully switch over from my Fairphone 6 though.)

Wait! Sorry.

Uh, I think you're talking about the Jolla C2 specifically. I can report that the Sony Xperia 10mk3 with SailfishOS works fine. Not sure about the C2.

https://commerce.jolla.com/products/jolla-phone-sep-ii-2026

Q:

Will the Jolla Phone work outside Europe, can I use it e.g. in the U.S.?

A:

Yes, we have designed the cellular band configuration to enable global traveling as much as possible, including e.g. roaming in the U.S. carrier networks.

4G & 5G global roaming modem configuration:

• LTE FDD: 1, 2, 3, 4, 5, 7, 8, 12, 17, 18, 19, 20, 25, 26, 28AB, 66
• LTE TDD: 34, 38, 39, 40, 41
• 5G NR: n1, n2, n3, n5, n7, n8, n12, n20, n26, n28, n38, n40, n41, n66, n77, n78

Back when VoLTE launched, they ran tests, and ever T-Mo based MVNO and T-Mo itself worked just fine.